When an image forming apparatus of an electrophotographic method adopts a direct transfer system of transferring an image by bringing a transfer member into contact with a photoconductor, the transfer member uses a conductive rubber roller (transfer roller) having a conductive shaft to rotate and drive the transfer member while matching the process speed of the photoconductor. A voltage applied to the transfer member is a DC bias voltage. At this time, the polarity of the DC bias voltage is identical to that of a transfer voltage for general corona discharge.
To achieve satisfactory transfer using the transfer roller, a voltage of generally 3 kV or more (the required current is several μA) must be applied to the transfer roller. This high voltage necessary for the image forming process is conventionally generated using a wire-wound electromagnetic transformer. The electromagnetic transformer is made up of a copper wire, bobbin, and core. When the electromagnetic transformer is used in the above specification, the leakage current must be minimized at each portion because the output current value is as small as several μA. For this purpose, the windings of the transformer must be molded with an insulator, and the transformer must be made large in comparison with supply power. This inhibits downsizing and weight reduction of a high-voltage power supply apparatus.
In order to compensate for these drawbacks, it is proposed to generate a high voltage by using a flat, light-weight, high-output piezoelectric transformer. By using, for example, a piezoelectric transformer formed from ceramic, the piezoelectric transformer can generate a high voltage more efficiently than in the use of the electromagnetic transformer. Since electrodes on the primary and secondary sides can be spaced apart from each other regardless of coupling between the primary and secondary sides, no special molding is necessary for insulation, thus making a high-voltage generation apparatus compact and lightweight.
Unfortunately, the high-voltage power supply apparatus using the conventional piezoelectric transformer cannot sometimes control the output voltage, so the circuit operation oscillates. Such a phenomenon degrades printing quality. That is, it is difficult to simply adopt, as a power supply unit in an image forming apparatus, the high-voltage power supply apparatus using the conventional piezoelectric transformer. Hence, it is demanded to realize stable voltage control free from any circuit oscillation.